ULTRASTABLE SYSTEM

a term developed by Ashby and defined by
him as follows: Two systems of continuous variables (that we
called 'environment' and 'reacting part') interact, so that a
primary feedback (through complex sensory and motor channels)
exists between them. Another feedback, working intermittently
and at a much slower order of speed, goes from the environment to
certain continuous variables which in their turn affect some
step-mechanisms, the effect being that the step-mechanisms change
value when and only when these variables pass outside given
limits. The step-mechanisms affect the reacting part; by acting
as parameters to it, they determine how it shall react to the
environment.

We can now appreciate how different an ultrastable system is
from a simple system when the conditions allow the difference to
show clearly. The difference can best be shown by an example.
The automatic pilot is a device which, amongst other actions,
keeps the airplane horizontal. It must, therefore, be connected
to the ailerons in such a way that when the plane rolls to the
right, its output must act on them so as to roll the plane to the
left. If properly joined, the whole system is stable and
self-correcting: it can now fly safely through turbulent air for,
though it will roll frequently, it will always come back to the
level. The Homeostat, if joined in this way, would tend to do
the same. (Though not well suited, it would, in principle, if
given a gyroscope, be able to correct roll.) So far, after a
small disturbance; but connect the ailerons in reverse and
compare them. The automatic pilot would act, after a small
disturbance, to INCREASE the roll and would persist in its wrong
action to the very end. The Homeostat, however, would persist in
its wrong action only until the increasing deviation made the
step-mechanisms start changing. On the occurrence of the first
suitable new value, the Homeostat would act to stabilize instead
of to overthrow; it would return the plane to the horizontal; and
it would then be ordinarily self-correcting for disturbances.
There is therefore some justification for the name 'ultrastable';
for if the main variables are assembled so as to make their field
unstable, the ultrastable system will change this field till it
is stable. The degree of stability shown is therefore of an
order higher than that of the system with a single field.
(Ashby, l960, pp. 98, l08)